When supplying power to a series connection of LEDs, achieving a stable current or power through the series connection is typically desirable. Conventionally power supplies for LEDs are implemented as buck converters, in which the current through the LEDs is determined by measuring a current through the main switch of the converter. When using a buck converter, however, the number of LEDs in series may be limited by the input voltage of the converter. At low input voltages, only a few LEDs can be connected in series.
In order to have more LEDs in series even at low input voltages, a boost converter may be used for driving the series connection. Control of a main switch of a boost converter may be implemented by using a dedicated pulse-width modulation (PWM) controller IC. The controller controls the main switch to conductive and non-conductive states in such a duty ratio that a desired output current is generated.
The controller IC may include a comparator configured to limit current through the main switch, for example. The current limiting function may operate by limiting the duty ratio of the PWM of the main switch. The ICs may also include a comparator for limiting the output voltage. If the output voltage is too high, the comparator may disable the PWM until the output voltage decreases to an acceptable level, for example.
Determining the current through the LEDs in a boost converter is not as simple as in a buck converter, since the output voltage also affects the current of the main switch. Instead, the current through the LEDs may be determined more directly by using a measurement resistor in series with the LEDs.
However, the level of internal reference voltage(s) for the comparator(s) is typically around 1.25 V. Thus, the feedback signal representing the measured current may have to be at this level. Such a voltage over the current measurement resistor may lead to significant power losses in the resistor.
In order to avoid excessive power losses in the measurement resistor, a measurement with a smaller resistance may be used and the voltage over the measurement resistor may be amplified. FIG. 1 shows an exemplary boost converter, where the measured voltage is amplified by using an operational amplifier. The boost converter is used to drive a series connection of LEDs 10. The main switch Q1 of the converter is controlled by a PWM controller IC U1. The controller U1 comprises a comparator and an internal reference voltage vref.
In FIG. 1, the voltage over a current measurement resistor R1 is amplified by using an amplifying circuitry 11 comprising an operational amplifier U2. The amplifying circuitry 11 also acts as a low-pass filter for the measured voltage. The circuitry 11 brings the current measurement to a voltage level that can be used in the controller U1. However, this implementation requires the use of relatively complex circuitry comprising a sufficiently accurate operational amplifier, which may make the implementation less cost-effective.